Solubilisation of carbonates by recycling condensates during digestion of monohydrate bauxites under pressure

ABSTRACT

A method for the production of aluminum from bauxites containing aluminum monohydate of the diaspore or boehmite type which may contain more than 1.5% by weight of calcium carbonates. The ore is digested by the following steps: a1) a suspension, produced from the milled bauxite ore and the concentrated aluminate liquor is fed to a plant containing a series of n autoclaves and a series of m regulators supplying m tubular reheaters across m autoclaves with steam; a2) the suspension first travels across the series of autoclaves, and on exiting the last autoclave A, the digestion of the ore is practically complete and a3) the suspension then travels across the series of regulators, the condensation water from the steam V coming from the regulator D and feeding the reheater R is collected in a purge pot P. In at least one position in the series of regulators, the suspension is diluted on entering a regulator with the water coming from the condensation of the steam from one or several upstream regulators.

TECHNICAL DOMAIN

The invention relates to an alumina production process according to theBayer process from bauxites containing alumina monohydrate of thediaspore or boehmite type, such as Greek and Chinese bauxites, which maycontain more than 1.5% by weight of calcium carbonates.

STATE OF THE ART

The Bayer process forms the main technique for the production of aluminato be transformed into aluminium by igneous electrolysis (called“metallurgical” alumina). It may also be used to manufacture a largeproportion of aluminas intended for the production of refractorymaterials, ceramics, grinding media, thermal insulation fillers,concretes, etc. (called “non-metallurgical” aluminas). According to thisprocess, the bauxite ore is treated while hot using an aqueous solutionof sodium hydroxide of an appropriate concentration, thus causingsolubilisation of the alumina and giving a liquor supersaturated withsodium aluminate (pregnant liquor).

The liquor must be raised to a high enough temperature to dissolve thealumina contained in the monohydrate bauxites: more than 220° C., andeven up to 250° C. to 270° C. The ore must be digested in autoclavescapable of resisting high pressures, typically several tens of bars.After separation of the solid phase that makes up the undigested residue(red mud) of the ore, the liquor supersaturated with sodium aluminate iscrystallised by seeding with aluminium trihydroxide particles in orderto cause precipitation of the alumina in solution in the form ofaluminium trihydroxide. The sodium aluminate liquor, depleted in aluminaafter crystallisation (spent liquor), is then recycled to digest thebauxite after being reconcentrated to an appropriate content of sodiumhydroxide (addition of hydroxide and/or evaporation).

Since the Bayer process is cyclic, any impurity derived from bauxite andintroduced into the liquor during digestion of the ore could accumulateif it is not periodically eliminated. This is the case particularly forsodium carbonate in solution in the Bayer liquor, for which theproportion by weight expressed by the ratio (carbonated Na₂O)/(causticNa₂O+carbonated Na₂O) must generally be approximately 10% (caustic Na₂Obeing free Na₂O+Na₂O in the form of aluminate AlO₂Na in solution). Abovea threshold of about 14%, sodium carbonate can precipitate unexpectedlyand reduce the productivity of the Bayer line installations. This is thecase particularly for flash tanks in the hot high pressure digestionequipment for bauxites rich in alumina monohydrate.

FIG. 1 shows part of the installation of a Bayer circuit according toprior art, corresponding to a continuous digestion workshop of bauxiteswith alumina monohydrates: bauxite (bx), when ground (grinder B) and inthe presence of an aliquot (L1) of the green liquor, forms a paste thatis mixed with the remainder of the green liquor (L2). The slurryresulting from the mix is pumped by powerful pumping means through aseries of n mechanically stirred digesters (A₁, . . . , A_(k−1), A_(k),A_(k+1), . . . , A_(n)) followed by a series of m flash tanks (D₁, . . ., D_(i−1), D_(i), D_(i+1), . . . , D_(m)) where m is less than n, beforereaching the dilution station (C) then the settlement station atatmospheric pressure (F).

As the slurry passes through each digester A_(k), it circulates close toa tubular heater (R_(k)) that passes through the digester (A_(k)). Thefirst m tubular heaters (R₁, . . . , R_(k−1), R_(k), R_(k+1), . . . ,R_(m)) are supplied by steam retrieved from the flash tanks (D_(m), . .. , D_(i+1), D_(i), D_(i−1)) and the next (m−n) tubular heaters(R_(m+1), . . . , R_(n)) are supplied by live steam (V) from the heatingplant. The temperature reached by the slurry at the output from eachdigester (A_(k)) is fixed in advance and is controlled by varying thesteam quantity (V_(i)) output from a flash tank (D_(i)) and passingthrough the heater (R_(k)). There is a state associated with each flashtank through which the slurry passes, characterised by an averagepressure level and temperature range which are relatively stable overtime. Industrial installations comprise a variable number n of digestersand a variable number m of flash tanks. A few years ago, n was typicallyclose to 10 and m was close to 7; at the moment the values areapproximately 20 for n and 12 for m.

The duration of the path followed by the slurry is such that the ore iscompletely digested at the exit from the last digester (A_(n)), wherethe temperature is close to 260° C., in other words most of the aluminacontained in the bauxite is dissolved in the aluminate liquor. Theslurry is then directed towards a series of flash tanks (D₁, . . . ,D_(i−1), D_(i), D_(i+1), . . . , D_(m)), each flash tank Di releasingsteam V_(i) supplying a heater R_(k) described above. At the exit fromthe last flash tank (D_(m)), the slurry is carried towards a station (C)where it is diluted before being sent to settlement tanks (F) forseparation of insoluble residues. The condensation water from steam (Vi)output from the flash tank (D_(i)) passing through the heater (R_(k)) toheat the slurry in the digester (A_(k)) is collected in a purge pot(P_(k)). Uncondensed steam (V′i) is connected to the steam supply(V_(i+1)) of the heater (R_(k−1)), and the condensate (Ei) contained inthe purge pot (P_(k)) is directed to the purge pot (P_(k−1)). Wateroutput from the purge port (P₁), called alkaline condensates (EDS) isused as industrial water, particularly for washing insoluble residuesextracted by settlement in the next workshop (F).

Monohydrate bauxites of the diaspore or boehmite type encounteredfrequently in Greece and in China are particularly rich in calciumcarbonate (CaCO₃>1.5%). During digestion, a large increase in thecontent of sodium carbonate is observed due to the addition of CO₃ ²⁻ions released during the dissociation of CaCO₃. This content ofcarbonates in the aluminate is usually higher than the content due tothe addition of carbonates alone into the bauxite, since lime is addedto improve the digestion efficiency, and the lime used itself includessome calcium carbonate particles, often for economic supply reasons.

Therefore, if the proportion by weight expressed by the (carbonatedNa₂O)/(caustic Na₂O+carbonated Na₂O) ratio exceeds a threshold of about14% (which depends particularly on the temperature of the liquor and itsconcentration in caustic Na2O), the sodium carbonate can precipitateunexpectedly and can disturb operation of the installations. The problemis particularly sensitive for flash tanks, one obvious problem beingmassive scaling at the input to one or several pipes (Ti) through whichthe slurry passes from one flash tank (D_(i)) to the next (D_(i+1)). Inthis case, the series has to be stopped to clean the clogged circuits.This operation in itself is long, difficult and expensive. It alsoblocks production, which significantly reduces the economic efficiencyof the BAYER line.

Patent application FR 1 523 304 (PECHINEY and KESTNER) recommends aprocess for bauxites with a high calcium carbonate content, in which afraction of the spent aluminate liquor, drawn off just aftercrystallisation and treated in a separate workshop including a line ofevaporators and a line of flash tanks, is oversaturated with sodiumcarbonate, such that sodium carbonate will crystallise with theappearance of coarse grains easy to separate from the aluminate liquor.The sodium carbonate is then extracted and caustified by lime afterredissolution. Unfortunately, this decarbonation process is veryexpensive in terms of investment and steam energy.

In French application FR 2 328 660, the applicant proposed a processalso adapted to the treatment of bauxites with a high calcium carbonatecontent (CaCO3>1.5%) such as diasporic bauxites from Greece or China.This process consists of trapping carbonate ions by precipitation ofbarium carbonate in a fraction of the spent liquor. The barium carbonateis then calcinated in the presence of aluminium trihydroxide Al(OH)3 togenerate the barium aluminate that is recycled. This process iseffective in eliminating carbonates but has three disadvantages:

-   -   it consumes large quantities of energy; of the order of 20 kg of        low sulphur content fuel per tonne of alumina    -   it significantly reduces production corresponding to the        aluminium trihydrate that must be continuously assigned to the        barium aluminate regeneration loop and the quantity of which        depends on the content of calcium carbonate in the bauxite    -   barium salts create difficult-to-solve environmental and hygiene        problems.

Finally, PECHINEY and the SOCIETE DES BAUXITES DU PARNASSE proposedanother process for Greek diasporic bauxites. This process consists ofgrinding the bauxite and then separating the carbonates by flotation andpartially eliminating the carbonates before injecting the said bauxiteinto the Bayer circuit (for example see the article published in TMS 99:“Alumina Production From Diasporic Bauxite” by E. Lavalou, O. Keramindasand B. Bosca). This type of process called “decalcitation” is efficientbut it requires the installation of a large auxiliary workshop on theinput side of the Bayer installations. The financial depreciation ofsuch an installation increases the manufacturing cost of alumina withthis type of bauxites.

PROBLEM THAT ARISES

The applicant attempted to develop an alumina production processaccording to the Bayer process from bauxites containing aluminamonohydrate of the diaspore or boehmite type, the said bauxites possiblycontaining more than 1.5% by weight of calcium carbonates. This processmust not have the economic and environmental disadvantages mentionedabove.

PURPOSE OF THE INVENTION

The purpose of the invention is an alumina production process frombauxites containing alumina monohydrate of the diaspore or boehmite typewith more than 1.5% by weight of calcium carbonate with the followinggeneral steps of a Bayer process:

-   -   a) digestion of the ore at a temperature of more than 220° C.        using an aluminate liquor with a soda concentration of more than        200 g of Na2O/litre;    -   b) dilution of the aluminate slurry thus obtained,    -   c) separation of the insoluble residues by settling and        filtering;    -   d) crystallisation in the liquor by seeding with aluminium        trihydroxide particles in order to cause precipitation of the        alumina in solution in the form of alumina trihydrate;    -   e) recycling of the spent sodium aluminate liquor to the bauxite        digestion step after having restored an appropriate        concentration of the sodium hydroxide,    -   the ore digestion step being carried out as follows:    -   a1) a slurry, made with ground bauxite ore and concentrated        alumina liquor, is transferred into an installation comprising a        series of n digesters (A₁, . . . , A_(k−1), A_(k), A_(k+1), . .        . , A_(n)) and a series of m flash tanks (D₁, . . . , D_(i−1),        D_(i), D_(i+1), . . . , D_(m)), where m is less than n,        supplying tubular heaters (R_(m), . . . , R_(k+1), R_(k),        R_(k−1) . . . , R₁) with steam passing through m digesters        (usually but not necessarily the first m digesters).    -   a2) the slurry passes firstly through the series of digesters        (A₁, . . . , A_(k−1), A_(k), A_(k+1), . . . , A_(n)), the        duration of its path and the temperature reached being defined        such that ore digestion is practically complete at the exit from        the last digester (An);    -   a3) the slurry then passes through the series of flash tanks        (D₁, . . . , D_(i−1), D_(i), D_(i+1), . . . , D_(m)) , the steam        Vi output from each flash tank D_(i) supplying the heater        (R_(k)) passing through the digester (A_(k)),    -   a4) the slurry is finally directed towards the dilution        station (C) and the settlement workshop (F);    -   the said process being characterised in that the slurry (S_(i))        entering a flash tank (D_(i+1)) is diluted at at least one        location along the line of the said flash tanks (D₁, . . . ,        D_(i−1), D_(i), D_(i+1), . . . , D_(m)) using water derived from        condensation of steam originating from one or several upstream        flash tanks (D₁, . . . , D_(i))

The “at least one location” along the line of flash tanks is preferablythe flash tank concerned by massive precipitation of sodium carbonatesor part of the pipe through which the slurry passes before arriving inthe said flash tank, close to the said flash tank. Therefore, thedilution water is brought directly into the flash tank concerned and/orinto the circulation tubing of the slurry on the input side of the flashtank.

The expression “entering a flash tank” means that the dilution may bedone either directly in the flash tank with a separate pipe opening upinto the flash tank and adding the water necessary for dilution, or by abranch connection made on the pipe in which the slurry is circulating onthe input side of the flash tank. In this case, the branch connection islocated close to the flash tank such that there is a significantpressure loss between this location and the output from the flash tankimmediately upstream, for example corresponding to at least 90% of thetotal pressure loss between the two flash tanks.

Let (D_(i)) be the flash tank concerned by the unexpectedprecipitations. According to the invention, the slurry (S_(i)) that iscollected in the flash tank (D_(i)) or the slurry (S_(i−1)) circulatingin the tubing (T_(i−1)) to reach the flash tank (D_(i)) is diluted withwater originating from one or more condensates originating from theupstream flash tanks. A separate pipe (G_(i)) carries this water eitherdirectly into the flash tank (D_(i)) that collects the slurry (S_(i)),or into the tubing (T_(i−1)) that carries the slurry (S_(i−1)) in theflash tank (D_(i)), the branch connection being located close to theflash tank (D_(i)). These two dilution possibilities may also be usedsimultaneously.

The applicant thought that instead of attempting to eliminate carbonatesat all costs, or at least significantly reducing their concentration byexpensive means according to prior art, it might be sufficient toprevent the precipitation of carbonates at sensitive locations, in otherwords at flash tank outlet tubings. This type of process makes itpossible to work with a proportion by weight expressed as the(carbonated Na₂O)/(caustic Na₂O+carbonated Na₂O) ratio more than about14%. This high content is an equilibrium content; evacuation ofcarbonates, sufficient to prevent their accumulation, is possible duringextraction of solid residues. While this is a slight drawback(particularly concerning productivity), it does not prevent efficientoperation of the Bayer process, provided that the unexpectedprecipitation of sodium carbonate and possibly other salts, particularlybased on sulphur, fluorine, phosphorus, arsenic and/or vanadium, such asvanadates, fluoro-arsenio-vanadates, etc., can be prevented.

For example FR 1 386 328 (PECHINEY) or FR 1 523 304 patent applicationsshow that the solubility of carbonates reduces with the causticconcentration of the liquor. Therefore, it is not surprising that thedilution provides a means of avoiding the precipitation of carbonates.However, the addition of water into the Bayer circuit is notparticularly desirable for economic reasons since this additionnecessarily degrades the energy balance; either more energy has to beconsumed to evaporate the water added in this way, or if water is takenout of the circuit itself, then it must be acceptable to work with amuch lower quantity of mud washing water (which results in an increasedsoda loss). The solution according to the invention is different fromsolutions according to prior art, which have all chosen the oppositeapproach; to concentrate the liquor at a precise location in the Bayercircuit, this location being chosen to obtain a controlled precipitationof impurities.

Moreover, the addition of water at such a location introduces a certainrisk since in a way it consists of a premature dilution that anticipatesthe planned dilution before settlement and can cause reversion, in otherwords premature precipitation of trihydrate in settlement tanks. Infact, surprisingly, tests carried out by the applicant show thatreversion risks are not significantly higher when limited dilution isdone in a flash tank.

Therefore, the invention consists of diluting the slurry to prevent theprecipitation of carbonates; the slurry (S_(i−1)) entering the flashtank (D_(i)), or the slurry (S_(i)) collected in the flash tank (D_(i))is diluted with water output from the condensation of steam originatingfrom one or several upstream flash tanks (D₁, . . . , D_(i−1)).Preferably, water derived from condensation of steam (V_(i−2))originating from the upstream flash tank (D_(i−2)) is used.

In one preferred embodiment of the invention, a system for collection ofwater from the condensation of steam derived from flash tanks andpassing through the heaters installed in the digesters, is used. Thiswater called alkaline condensate, is collected in purge pots, one purgepot (P_(k)) collecting firstly water from upstream purge pots (P_(m), .. . , P_(k+1)) and secondly the condensate (E_(i)) of steam (Vi) outputfrom flash tank (D_(i)).

Preferably, the water that will be used to dilute the slurry (S_(i))collected in a flash tank (D_(i)) and/or the slurry (S_(i−1)) enteringthe flash tank (D_(i)) is drawn off at the output from the purge pot(P_(k+2)). It is carried through a pipe (G_(i)) either directly into theflash tank (D_(i)) (duct g_(ia)), or into the pipe (T_(i−1)) throughwhich the slurry (S_(i−1)) circulates on the upstream side of the flashtank (D_(i)) (duct g_(ib)).

Similarly, temperature differences between the slurry and the water thatwill be diluted are minimum, which facilitates maintaining thecarbonates in a stable dissolved state. The “water” balance is notaffected very much; admittedly, less water exits from the purge potcircuit (distilled soda water) but water is taken from the circuititself and is re-added fairly quickly, in a downstream flash tank,anticipating some of the dilution performed on the downstream side ofthe last flash tank (station C) and that will control the requiredcaustic concentration of the slurry at the inlet to the first settlementtank (workshop F). Therefore, this reduces water consumption at thedilution station (C), the water thus saved possibly for example beingused to compensate for the smaller addition of distilled soda water tothe mud washing water. This also reduces soda consumption since sodaentrained by the flash steam is evacuated in smaller quantities with thecondensates used for washing mud.

It is preferred to use condensate from the second flash tank upstreamfor dilution, since the pressure in the pot that collects the condensatefrom the flash tank directly on the upstream side is approximately equalto the pressure in the flash tank concerned, such that the addition ofwater originating from the pot that collects the condensate from theflash tank directly on the input side needs to be assisted by the use ofan additional pump.

Moreover, diluting with alkaline condensates consisting of recycledprocess hot water, provides a means of improving the thermal efficiencywith respect to a dilution that would be made with water which wouldrequire an additional energy cost for specific heating.

Since the massive carbonate precipitation phenomenon occurs within anarrow temperature range, in principle a single flash tank should beconcerned. However, normal variations of control parameters ofindustrial installations, variations of the carbonate content of thebauxite being treated and possibly frequent modifications imposed in thesupply of bauxites, and possibly the precipitation of other salts atdifferent temperatures that could concern other flash tanks, are allreasons for providing as many means of diluting the slurry as possible,the best method being to provide dilution at the input to every flashtank. This dilution is preferably made with the condensate from thesecond flash tank upstream.

Moreover, the fact of recycling alkaline condensates at severallocations provides a means of minimising energy consumption. Byrecycling the said alkaline condensates that is at a temperature closeto the temperature of the slurry to be diluted, the result is to act ona medium that is close to the state of thermodynamic reversibility sincelarge temperature differences between the fluids present are avoided. Asthe number of dilution points increases, the phenomenon becomes closerto perfect reversibility and the energy consumption becomes lower. Thus,the necessary quantity of water is injected in a maximum number of flashtanks.

Therefore, preferably each outlet tubing from a purge pot (P_(k+2)) isequipped with a branch connection (G_(i)) through which part of thewater thus drawn off may be used to dilute the slurry (S_(i)) collectedin the flash tank (D_(i)) or the slurry (S_(i−1)) entering the flashtank (D_(i)). Moreover, a valve fitted on each branch connection (G_(i))can be used to control the flow of dilution water; sufficiently dilutionwater is necessary to prevent precipitation of carbonates, but too muchshould be avoided to prevent extra energy losses and possibly reversion.

A large number of pipes (Gi) is particularly desirable if several othersalts (based on sulphur, fluorine, phosphorus, arsenic and/or vanadiumsuch as vanadates, fluoro-arsenio-vanadates, etc.) are likely toprecipitate suddenly and unexpectedly in the flash tank circuit, buteach within a distinct temperature range corresponding to a specificflash tank, which may be different from the flash tank concerned byprecipitation of carbonates. It is thus easier to control the risk ofprecipitation of each of these salts separately.

Finally, and economically, the installation of one or several additionalpipes results in a negligible extra investment cost, incomparably lowerthan the cost of equipment envisaged in prior art (barium carbonateprecipitation line and recycling of barium aluminate, bauxitedecalcitation workshop).

The quantity of water used to dilute the slurry to prevent a massiveprecipitation of carbonates and other salts may be determined preciselyfrom solubility curves for these salts, particularly form carbonates.

FIG. 3 illustrates a diagrammatic curve as an example, showing thelimiting concentration at a given temperature of carbonates expressed interms of carbonated soda (carbonated Na2O), varying as a function of thecaustic concentration of the liquor. The water quantity necessary fordilution is approximately proportional to the distance between the saidcurve and the operating point at the location of the flash circuitconsidered.

Thus, for any pipe G_(i), it is possible to add water to dilute theslurry with a flow determined as a function of the temperature and thecarbonate concentration in the slurry reaching the inlet to the flashtank considered, by adjusting the valve installed on the said pipe.

Particular Embodiment of the Invention (Global Installation Illustratedin FIG. 1 with n=19 and m=12; Use of Dilution according to the Inventionwith i=9 and k=4)

In this example, the diasporic bauxite to be treated is a Chinesebauxite containing 4.3% of calcium carbonates.

The continuous digestion workshop comprises 19 digesters and 12 flashtanks. The slurry output from the last digester is at 265° C. at 50bars.

Without the bypass according to the invention, fast clogging of flashtank D9 occurs. The circuit tends to get blocked after a few days ofoperation at a point in the circuit at which the solution temperature isapproximately 150° C.

If there is no dilution, the caustic concentration of slurry S8 at theinlet to flash tank D9, is 260 g/l expressed in total Na2O and thecarbonate content expressed as (carb Na2O/total Na2O) is equal to 14%.The corresponding operating point is remote from the solubilisationcurve in FIG. 3 and the distance d from this point to the solubilisationcurve indicates that slurry S8 circulating at a flow rate of 375 m3/hwill have to be diluted with a flow rate of 10 m3/h at the entry toflash tank D9 to prevent the precipitation of carbonates at thislocation, particularly at the exit from flash tank D9.

If water drawn from the outside is used for dilution at a temperature150° C., precipitation is prevented but there is an energy increase ofthe order of 133 MJ/t of alumina produced.

However, if some of the water output from purge pot P6 is drawn off,itself supplied by water output from upstream purge pots P12 to P7 andsteam condensate V7 output from upstream flash tank D7 to pass throughheater R6 of digester A6. This water is injected through pipe G9 at aflow rate of 10 m³/h into slurry S8 circulating in tubing T8, the branchconnection of pipe g_(9b) in tubing T8 being located close to the inletinto flash tank D9 (approximately 100 mm).

It is found that massive scaling no longer occurs in the tubings at theexit from flash tank D9.

Advantages

-   -   possibility of preventing precipitation of other salts    -   particularly attractive for diasporic bauxites already        containing carbonates, this process can be used for other types        of bauxites, knowing that the general trend in the Bayer process        is to increase the caustic concentration of aluminate liquor in        order to improve productivity and that the control of the Bayer        process will increasingly be affected by problems of unexpected        precipitations of carbonates (vanadates and other salts        described above) caused by their lower solubility in these        media.

1. Alumina production process from bauxites containing aluminamonohydrate of the diaspore or boehmite type with more than 1.5% byweight of calcium carbonate with the following general steps of a Bayerprocess: a) digestion of the ore at a temperature of more than 220° C.using an aluminate liquor with a soda concentration of more than 200 gof Na2O/litre; b) dilution of the aluminate slurry thus obtained, c)separation of the insoluble residues by settling and filtering, d)crystallisation of the liquor by seeding with aluminium trihydroxideparticles in order to cause precipitation of the alumina in solution inthe form of alumina trihydrate; e) recycling of the spent sodiumaluminate liquor to the bauxite digestion step after having restored anappropriate concentration of the sodium hydroxide, the ore digestionstep in the said process being carried out as follows: a1) a slurry,made with ground bauxite ore and concentrated alumina liquor, istransferred into an installation comprising a series of n digesters (A₁,. . . , A_(k−1), A_(k), A_(k+1), . . . , A_(n)) and a series of m flashtanks (D₁, . . . , D_(i−1), D_(i), D_(i+1), . . . , D_(m)) where m isless than n, supplying tubular heaters (R_(m), . . . , R_(k+1), R_(k),R_(k−1) . . . , R₁) with steam passing through m digesters. a2) theslurry then passes firstly through the series of digesters (A₁, . . . ,A_(k−1), A_(k), A_(k+1), . . . , A_(n)), the duration of its path beingdefined such that ore digestion is practically complete at the exit fromthe last digester An; a3) the slurry then passes through the series offlash tanks (D₁, . . . , D_(i−1), D_(i), D_(i+1), . . . , D_(m)), thesteam (Vi) output from each flash tank (D_(i)) supplying the heater(R_(k)) passing through the digester (A_(k)), a4) the slurry is finallydirected towards the dilution station (C) and the settlement workshop(F); the said process being characterised in that the slurry (S_(i−1))entering a flash tank (D_(i)) is diluted at at least one location alongthe line of the said flash tanks (D₁, . . . , D_(i−1), D_(i), D_(i+1), .. . , D_(m)), using water derived from condensation of steam originatingfrom one or several upstream flash tanks (D₁, . . . , D_(i+1)). 2.Process according to claim 1, in which the slurry (S_(i−1)) entering aflash tank (D_(i)) or the slurry (S_(i)) collected in the flash tank(D_(i)), or both, is (are) diluted, using water derived fromcondensation of steam originating from the upstream flash tank (D_(i−2))3. Process according to claim 2, in which dilution water is drawn off atthe output from the purge pot (P_(k+2)) that collects the condensate(E⁻²) from the steam (V_(i−2)) derived from the flash tank (D_(i−2)) andis inserted in the slurry (S_(i)) using a pipe (G_(i)) that connects theoutput pipe from the purge pot (P_(k+2)) to the flash tank (D_(i)). 4.Process according to claim 2, in which dilution water is drawn off atthe output from the purge pot (P_(k+2)) that collects the condensate(E_(i−2)) from the steam (V_(i−2)) derived from the flash tank (D_(i−2))and is inserted in the slurry (S_(i−1)) using a pipe (G_(i)) thatconnects the output pipe from the purge pot (P_(k+2)) to the pipe(T_(i−1)) through which the slurry (S_(i−1)) circulates, through abranch connection located close to the flash tank (D_(i)).
 5. Processaccording to claim 3, in which the purge pot (P_(k+2)) also collectswater from the upstream purge pots (P_(k+3), . . . , P_(m)).
 6. Processaccording to claim 3, in which a pipe (G_(i)) connects the output pipefrom each purge pot (P_(k+2)) to each flash tank (D_(i)).
 7. Processaccording to claim 1, in which the dilution water flow is determinedusing a solubilisation curve defined in advance.
 8. Process according toclaim 7, in which the dilution water flow is controlled using a valveinstalled on the pipe (G_(i)).